Spacecraft in earth and solar orbit are subject to constant bombardment by charged particles of both positive and negative polarity. The energy of the charged particles bombarding the spacecraft ranges from very low to extremely high values, i.e, from zero electron volts to 10.sup.10 electron volts, in some instances. In response to the charged particle bombardment, the spacecraft exterior surface acquires a potential relative to an ambient plasma through which the spacecraft is traveling. The potential causes the net electric current flow between the spacecraft surface and the ambient plasma to be zero, whereby the numbers of unit positive and negative charges flowing between the plasma and the spacecraft current per unit time are exactly equal. In response to the positive and negative charges flowing between the spacecraft and the ambient plasma, an equilibrium potential is reached or induced on the surface of the spacecraft. The equilibrium potential may be a function of orbital parameters of the spacecraft, solar illumination, solar activity, and the type of material on the spacecraft surface. The factors effecting the spacecraft equilibrium potential are the work function of metals on the spacecraft exterior surface and the nature of the particles bombarding the spacecraft surface. It is believed that the solar energy impinging on the spacecraft metal surface causes photoelectrons to be emitted by the surface, partially or totally compensating for energetic ambient electrons incident upon the spacecraft surface. This conclusion is based on the finding that spacecraft in synchronous earth orbit during eclipse sometimes have surface potentials of several thousand volts negative relative to the plasma through which they are traveling. In contrast, spacecraft in relatively low altitude earth orbit, i.e., earth orbits on the order of 100 miles, where the plasma densities are much greater and the particle energies are much lower, have variations that are typically .+-.5 volts.
In certain types of spacecraft, the induced voltages have caused different, electrically isolated segments of the spacecraft surface to assume different potentials. For example, if different, electrically isolated portions of a spacecraft surface, are differentially illuminated by the sun, different potentials are established between these different parts. The potential difference can become great enough to cause an electric discharge between the different segments, resulting in damage to electronic circuitry within the spacecraft. If the entire spacecraft surface is maintained at the same potential, e.g., at the potential of the plasma through which the spacecraft travels, the problem of electric discharge between the different parts of the spacecraft surface is avoided and the destruction of electronic components within the spacecraft due to the discharge is thereby obviated.
If a differential potential occurs between the spacecraft surface and the plasma through which a spacecraft is traveling, it has an adverse effect on instruments to measure the characteristics of charged particles encountered in spacecraft flight. In particular, such a potential difference has an effect on the trajectories of particles impacting on the instruments so that inaccurate indications of the particle trajectories quantities and other characteristics are likely to result.